class_docstr: Head of `VarifocalNet (VFNet): An IoU-aware Dense Object
  Detector.<https://arxiv.org/abs/2008.13367>`_.
  
  The VFNet predicts IoU-aware classification scores which mix the
  object presence confidence and object localization accuracy as the
  detection score. It is built on the FCOS architecture and uses ATSS
  for defining positive/negative training examples. The VFNet is trained
  with Varifocal Loss and empolys star-shaped deformable convolution to
  extract features for a bbox.
  
  Args:
      num_classes (int): Number of categories excluding the background
          category.
      in_channels (int): Number of channels in the input feature map.
      regress_ranges (tuple[tuple[int, int]]): Regress range of multiple
          level points.
      center_sampling (bool): If true, use center sampling. Default: False.
      center_sample_radius (float): Radius of center sampling. Default: 1.5.
      sync_num_pos (bool): If true, synchronize the number of positive
          examples across GPUs. Default: True
      gradient_mul (float): The multiplier to gradients from bbox refinement
          and recognition. Default: 0.1.
      bbox_norm_type (str): The bbox normalization type, 'reg_denom' or
          'stride'. Default: reg_denom
      loss_cls_fl (dict): Config of focal loss.
      use_vfl (bool): If true, use varifocal loss for training.
          Default: True.
      loss_cls (dict): Config of varifocal loss.
      loss_bbox (dict): Config of localization loss, GIoU Loss.
      loss_bbox (dict): Config of localization refinement loss, GIoU Loss.
      norm_cfg (dict): dictionary to construct and config norm layer.
          Default: norm_cfg=dict(type='GN', num_groups=32,
          requires_grad=True).
      use_atss (bool): If true, use ATSS to define positive/negative
          examples. Default: True.
      anchor_generator (dict): Config of anchor generator for ATSS.
  
  Example:
      >>> self = VFNetHead(11, 7)
      >>> feats = [torch.rand(1, 7, s, s) for s in [4, 8, 16, 32, 64]]
      >>> cls_score, bbox_pred, bbox_pred_refine= self.forward(feats)
      >>> assert len(cls_score) == len(self.scales)
docstr: Transform outputs for a single batch item into bbox predictions.
  
  Args:
      cls_scores (list[Tensor]): Box iou-aware scores for a single scale
          level with shape (num_points * num_classes, H, W).
      bbox_preds (list[Tensor]): Box offsets for a single scale
          level with shape (num_points * 4, H, W).
      mlvl_points (list[Tensor]): Box reference for a single scale level
          with shape (num_total_points, 4).
      img_shape (tuple[int]): Shape of the input image,
          (height, width, 3).
      scale_factor (ndarray): Scale factor of the image arrange as
          (w_scale, h_scale, w_scale, h_scale).
      cfg (mmcv.Config | None): Test / postprocessing configuration,
          if None, test_cfg would be used.
      rescale (bool): If True, return boxes in original image space.
          Default: False.
      with_nms (bool): If True, do nms before returning boxes.
          Default: True.
  
  Returns:
      tuple(Tensor):
          det_bboxes (Tensor): BBox predictions in shape (n, 5), where
              the first 4 columns are bounding box positions
              (tl_x, tl_y, br_x, br_y) and the 5-th column is a score
              between 0 and 1.
          det_labels (Tensor): A (n,) tensor where each item is the
              predicted class label of the corresponding box.
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code: def _get_bboxes_single(
      self,
      cls_scores,
      bbox_preds,
      mlvl_points,
      img_shape,
      scale_factor,
      cfg,
      rescale=False,
      with_nms=True,
  ):
      """Transform outputs for a single batch item into bbox predictions.
  
      Args:
          cls_scores (list[Tensor]): Box iou-aware scores for a single scale
              level with shape (num_points * num_classes, H, W).
          bbox_preds (list[Tensor]): Box offsets for a single scale
              level with shape (num_points * 4, H, W).
          mlvl_points (list[Tensor]): Box reference for a single scale level
              with shape (num_total_points, 4).
          img_shape (tuple[int]): Shape of the input image,
              (height, width, 3).
          scale_factor (ndarray): Scale factor of the image arrange as
              (w_scale, h_scale, w_scale, h_scale).
          cfg (mmcv.Config | None): Test / postprocessing configuration,
              if None, test_cfg would be used.
          rescale (bool): If True, return boxes in original image space.
              Default: False.
          with_nms (bool): If True, do nms before returning boxes.
              Default: True.
  
      Returns:
          tuple(Tensor):
              det_bboxes (Tensor): BBox predictions in shape (n, 5), where
                  the first 4 columns are bounding box positions
                  (tl_x, tl_y, br_x, br_y) and the 5-th column is a score
                  between 0 and 1.
              det_labels (Tensor): A (n,) tensor where each item is the
                  predicted class label of the corresponding box.
      """
      cfg = self.test_cfg if cfg is None else cfg
      assert len(cls_scores) == len(bbox_preds) == len(mlvl_points)
      mlvl_bboxes = []
      mlvl_scores = []
      for cls_score, bbox_pred, points in zip(
          cls_scores, bbox_preds, mlvl_points
      ):
          assert cls_score.size()[-2:] == bbox_pred.size()[-2:]
          scores = (
              cls_score.permute(1, 2, 0)
              .reshape(-1, self.cls_out_channels)
              .contiguous()
              .sigmoid()
          )
          bbox_pred = (
              bbox_pred.permute(1, 2, 0).reshape(-1, 4).contiguous()
          )
  
          nms_pre = cfg.get("nms_pre", -1)
          if 0 < nms_pre < scores.shape[0]:
              max_scores, _ = scores.max(dim=1)
              _, topk_inds = max_scores.topk(nms_pre)
              points = points[topk_inds, :]
              bbox_pred = bbox_pred[topk_inds, :]
              scores = scores[topk_inds, :]
          bboxes = distance2bbox(points, bbox_pred, max_shape=img_shape)
          mlvl_bboxes.append(bboxes)
          mlvl_scores.append(scores)
      mlvl_bboxes = torch.cat(mlvl_bboxes)
      if rescale:
          mlvl_bboxes /= mlvl_bboxes.new_tensor(scale_factor)
      mlvl_scores = torch.cat(mlvl_scores)
      padding = mlvl_scores.new_zeros(mlvl_scores.shape[0], 1)
      # remind that we set FG labels to [0, num_class-1] since mmdet v2.0
      # BG cat_id: num_class
      mlvl_scores = torch.cat([mlvl_scores, padding], dim=1)
      if with_nms:
          det_bboxes, det_labels = multiclass_nms(
              mlvl_bboxes,
              mlvl_scores,
              cfg.score_thr,
              cfg.nms,
              cfg.max_per_img,
          )
          return det_bboxes, det_labels
      else:
          return mlvl_bboxes, mlvl_scores
  
imports: import numpy
  import torch
  import torch.nn
  from mmcv.cnn import ConvModule, Scale, bias_init_with_prob, normal_init
  from mmcv.ops import DeformConv2d
  from mmcv.runner import force_fp32
  from mmdet.core import bbox2distance, bbox_overlaps, build_anchor_generator, build_assigner, build_sampler, distance2bbox, multi_apply, multiclass_nms, reduce_mean
  from ..builder import HEADS, build_loss
  from .atss_head import ATSSHead
  from .fcos_head import FCOSHead
label_desc: Use image node_id 9 for calling the VFNetHead obj's underlying member method code with example usage: obj._get_bboxes_single(cls_scores, bbox_preds, mlvl_points, img_shape, scale_factor, cfg, rescale, with_nms) and returns: det_bboxes,  det_labels, mlvl_bboxes,  mlvl_scores